Rechargeable or secondary electrochemical storage devices or batteries have wide-ranging applications and development of improved battery performance is a long-standing goal. Maximizing the volumetric or gravimetric energy density (i.e. minimizing the cell volume or mass) is an important and closely tracked performance metric. Rechargeable electrochemical cells such as Li-ion and NiMH use an electrochemically active, non-metallic, insertion material at the negative electrode or anode. However, many electrochemical storage systems involve the use of an electrochemically active metal at the anode. Commercial examples include Pb-acid, Na—NiCl2 (ZEBRA), Li metal polymer and Ag—Zn, but many other examples have been explored in the laboratory setting including Li—S, non-aqueous Na, and Mg.
In general, in all closed system or sealed container liquid-based cells, the cell is designed such that the capacity of the anode exceeds the capacity of the cathode.
For example in an Mg cell (Aurbach, D. et. al., Prototype systems for rechargeable magnesium batteries, Nature 407(2000), 724-727) the negative electrode is typically a metallic Mg foil or ribbon on the order of at least 100 μm thick, or 38 mAh/cm2; containing significant excess capacity relative to the cathode which is typically constructed at <5 mAh/cm2. Calculating the N/P ratio of these cells, where N and P are the areal electrochemical capacity of the negative and positive electrodes (measured in mAh/cm2), the Mg cells reported in the literature have N/P typically >10 and frequently >30. For further example in a Zn cell the negative electrode is typically a Zn metal foil or block. In still a further example in a Pb-acid cell the anode comprises a large block of Pb, always in significant excess capacity relative to the cathode.
Another report is Zheng, Y. et. al., Magnesium cobalt silicate materials for reversible magnesium ion storage, Electrochemica Acta, 66(2012), 75-81, which discloses a Magnesium battery having a solid Mg metal foil as the anode. The battery is built in the fully discharged state and has a thick Mg metal foil as the anode, giving an anode excess N/P>1.
In yet another report Liu, B. et. al., Rechargeable Mg-Ion Batteries Based on WSe2 Nanowire Cathodes, ACS Nano, 7(2013), 8051-80587, which discloses a Magnesium battery having a solid Mg metal foil as the anode. The battery is built in the fully charged state and has a thick Mg metal foil as the anode, giving an anode excess N/P>1.
Two classes of closed system Li cells seem especially relevant for the present discussion. In a standard Li-ion cell, Li ions are intercalated into, or shuttled between, both the cathode and anode. The anode may typically be graphite, although a range of other anode materials such as silicon, germanium, tin, aluminum, and alloys thereof are also well known. In addition, low voltage intercalation hosts, such as lithium titanium oxide (Li4Ti5O12 or “LTO”), and conversion materials, such as low voltage oxides, may be used as an anode. In all these cases it is well known in the art that it is necessary to design a cell with more reversible capacity at the anode than at the cathode. This is to ensure that during as-rated charging operation (i.e. transfer of lithium and electrons from cathode to anode at a designated rate) the anode may always accept more lithium than is removed from the cathode. Excess-anode devices constructed in this manner minimize the risk of plating lithium metal during charging which is widely believed to be detrimental to cell cycle life and safety. Thus lithium-ion cells are designed with an excess of reversible capacity on the negative electrode, typically denoted as an N/P ratio of >1. The required excess of negative electrode varies depending on the selected anode material, but may typically lie in the range 20-40% (for graphite) to 10% (for LTO). Thus in a lithium-ion battery it is well known that an N/P ratio>1 and general N/P>1.1 is required for good operation of the device.
In a lithium metal cell with a liquid or gel electrolyte, the anode is chosen to be a metal foil of lithium or a metallic alloy of Li such as LiAl, (a range of such anodes are well known). Examples of such cell chemistries include lithium-sulfur cells (Sion, Oxis), lithium-molybdenum disulfide (Moli) and lithium-vanadium oxide (Avistor, Valence, Batscap, Bollore, NTT). However lithium metal anodes are known to be highly reactive, which on cycling continuously generate high surface area lithium and decomposition products. The generation of high surface area metallic lithium and decomposition products lowers the onset of thermal instability leading to significant and well documented safety hazards. Additionally, metallic lithium anodes are known to rapidly lose accessible capacity through the formation of finely divided and electrically isolated regions of metallic lithium. Because of the reactivity of lithium, such lithium metal cells with liquid electrolyte are designed with a very large N/P ratio. The N/P ratio in a lithium metal cell is typically around 10, but in cases where the cell volume is minimized N/P may be around 4 (K. Brandt Solid State Ionics 69, (1994) 173-183 and Electrically Rechargeable Metal-air Batteries Compared to Advanced Lithium-ion Batteries, presented by Jeff Dahn at IBM Almaden Institute, 2009). Thus in any lithium metal cell having a liquid electrolyte, it is well known that an N/P ratio>>1 and in general N/P>4 is required for good operation of the device (i.e., useful cycle life and energy density).
Similar arguments to the forgoing also apply to open-system cells, such as Zn-air and Li-air and again the cells are designed with a metal anode that has much larger capacity than the capacity of the cathode electrode. While such open-system cells utilize air as an active material, the capacity of the cathode electrode (P) is well defined and limited to a finite value. Therefore in such cells N/P is again designed to be >1.
An exception to the N/P>1 rule would be a solid-electrolyte lithium-metal cell such as that reported in U.S. Pat. No. 6,168,884 B1 issued Jan. 2, 2001 to Neudecker et al., where it is generally known that the solid electrolyte has negligible reaction with the lithium metal anode, so that a cell may be designed which has an N/P ratio<1. Similar cells have subsequently been reported by numerous authors. For example, in Neudecker, a cell is shown having no lithium metal in the fully discharged state. Because the reaction with a solid electrolyte is negligible, the Neudecker all-solid cell can be cycled reversibly many times despite having an N/P ratio<1. However, the cell reported by Neudecker suffers from a prohibitively low electrode loading, generally less than 0.1 mAh/cm2, required to meet a practical rate capability.
In a similar fashion to Neudecker, other solid-electrolyte cells such as those described in U.S. Pat. No. 6,402,795 issued Jun. 11, 2002 to Chu et al. use a solid electrolyte barrier layer, also referred to as a passivation layer in conjunction with a liquid electrolyte. The barrier layer, deliberately coated onto the negative electrode prior to cell assembly, is required to prevent spontaneous and continued reaction of the anode material with the liquid electrolyte. However, all solid-electrolyte cells as well as hybrid barrier layer with liquid electrolyte cells suffer from major disadvantages in terms of manufacturability and rate performance.
Another report is U.S. Pat. No. 5,314,765, Protective lithium ion conducting ceramic coating for lithium metal anodes and associate method, issued May 24, 1994 to Bates, which is said to disclose a battery structure including a cathode, a lithium metal anode and an electrolyte disposed between the lithium anode and the cathode utilizes a thin-film layer of lithium phosphorus oxynitride overlying so as to coat the lithium anode and thereby separate the lithium anode from the electrolyte. If desired, a preliminary layer of lithium nitride may be coated upon the lithium anode before the lithium phosphorous oxynitride is, in turn, coated upon the lithium anode so that the separation of the anode and the electrolyte is further enhanced. By coating the lithium anode with this material lay-up, the life of the battery is lengthened and the performance of the battery is enhanced.
In summary, electrochemical systems that contain liquid or gel electrolyte, and not exclusively solid electrolyte, are designed either with an intercalation anode having an N/P ratio>1 and typically >1.2, or with a pure metal anode having an N/P ratio>1 and typically >4. This arises from a belief that plated metal has a poorly controlled morphology or undergoes spontaneous chemical reactions with electrolyte components and therefore it is advantageous to either have a large metal excess in order to counteract these processes, or to avoid plating metal altogether as for intercalation systems.
In a recent report “Electrically Rechargeable Metal-air Batteries Compared to Advanced Lithium-ion Batteries”, presented at IBM Almaden Institute, 2009 by Jeff Dahn, NSERC/3M Canada Industrial Research Chair, Depts. of Physics and Chemistry, Dalhousie University: Canada, which teaches practitioners not to use metallic Lithium in rechargeable cells. Additionally it is said to teach that excess lithium, or N/P>1, is a requirement for electrochemical cells utilizing a metal lithium anode. Specifically N/P=4 is required for useful cycle life.
An additional report is K. Brandt, Solid State Ionics 69, (1994) 173-183, which teaches that rechargeable Li batteries in general require N/P>1 for electrochemical cells.
In yet another report is Harry, Hallinan, Parkinson, MacDowell, and Balsara, Detection of subsurface structures underneath dendrites formed on cycled lithium metal electrodes, Nature Materials 2013, 13, 69-73 which is said to disclose that during the early stage of dendrite development, the bulk of the dendritic structure lies within the metal electrode, underneath the polymer/electrode interface. Furthermore, they observed crystalline impurities, present in the uncycled lithium anodes, at the base of the subsurface dendritic structures. The portion of the dendrite protruding into the electrolyte increases on cycling until it spans the electrolyte thickness, causing a short circuit. Contrary to conventional wisdom, it seems that preventing dendrite formation in polymer electrolytes depends on controlling the formation of subsurface structures in the lithium electrode present prior to cell assembly.
Yet another report is Vaughey et al., Lithium Metal Anodes, Annual Merit Review, DOE Vehicle Technologies Program, Washington, D.C., May 19, 2009, which is said to teach, inter alia, that cycled lithium metal anodes have a complex morphology that lies at the heart of the lifetime problems.
Another report is Mikhaylik, Protection of Li Anodes Using Dual Phase Electrolytes (Sion Power, DoE SERE report May 10, 2011), which is said to teach the protection of Li anode with dual phase electrolyte eliminated thermal runaway for 50% of the 0.25 Ah rechargeable Li—S cells tested at end of life.
Yet another report is Park, M. S., et. al. A highly reversible lithium metal anode. Nature Scientific Reports, 4, (2014), 3815, which is said to disclose a novel electrolyte system that is relatively stable against lithium metal and mitigates dendritic growth. A significant basis for the paper is a cell model in which N/P is 1.1 and 3 (i.e., N/P>1) for lithium ion and lithium metal cells respectively.
Another report is U.S. Pat. No. 6,706,447, Lithium Metal Dispersion In Secondary Battery Anodes, issued Mar. 16, 2004 to Gao et al., which is said to disclose a secondary battery having a high specific capacity and good cyclability, and that can be used safely. This document inter alia explicitly states the requirement that the amount of metal used in the battery should be chosen to be less than the amount that can be incorporated into the anode (i.e. N>P)
Yet another report is Li et al., A Review Of Lithium Deposition In Lithium-Ion And Lithium Metal Secondary Batteries, Journal of Power Sources 254 (2014) 168-182, which is said to disclose major aspects related to lithium deposition in lithium-ion and lithium metal secondary batteries are reviewed. For lithium-ion batteries with carbonaceous anode, lithium deposition may occur under harsh charging conditions such as overcharging or charging at low temperatures. The authors state that metal deposition is always disadvantageous, and that the solution includes ensuring that the battery design has a sufficiently large excess of anode or N/P>1.
Another report is U.S. Pat. No. 6,258,478 B1, Electrode Assembly Having A Reliable Capacity Ratio Between Negative And Positive Active Materials And Battery Having The Same, issued Jul. 10, 2001 to Kim, which is said to disclose a roll electrode assembly used in a secondary battery includes a positive electrode applied with a positive active material, a negative electrode applied with a negative active material, and a separator disposed between said positive and negative electrodes. A thickness of the positive or negative active materials applied on opposite sides of positive or negative substrates of the positive or negative electrodes are different from each other such that the capacity ratio between the positive and negative electrodes (N/P) can be maintained above 1.
Yet another report is U.S. Pat. No. 5,422,203, Disposing A Prepared Electrolyte Between The Electrodes, The Nonaqueous Electrolyte Comprising Of Lithium Tetrafluoroborate Lithium Hexafluorophosphate, Dimethyl Carbonate And Ethylene Carbonate, issued Jun. 6, 1995 to Guyomard et al., which is said to disclose that irreversible loss of lithium during the initial discharge cycle of secondary batteries with carbon intercalation electrodes is substantially reduced by employing as the cell electrolyte a non-aqueous solution of LiPF6 in a mixture of dimethylcarbonate and ethylene carbonate. By this means, in a secondary battery cell comprising, for example, a Mn2O4 positive electrode and a graphite negative electrode, up to about 90% of the theoretical level of lithium can be reversibly cycled at an exceptionally high rate of about CR (complete discharge in one hour).
In yet another report on non-aqueous electrolyte batteries with a negative electrode comprising lithium titanate (LTO), U.S. Pat. No. 7,883,797, issued Feb. 8, 2011 to Kishi et al. states “[a] non-aqueous electrolyte battery . . . has a positive electrode having a discharge capacity of 1.05 or more times that of a negative electrode thereof.” However, Kishi et al. explicitly recite at column 4, lines 52-60: “The discharge capacity of the aforementioned positive electrode is preferably 1.10 or less times that of the aforementioned negative electrode to prevent the extreme drop of the capacity of the entire battery and the potential of the negative electrode. In particular, the discharge capacity of the aforementioned positive electrode is more preferably from 1.05 to 1.07 times that of the aforementioned negative electrode to prevent the deterioration of the positive active material at a temperature as high as 60° C. or more.” The inverse of a ratio (i.e., the N/P ratio) of 1.10 to 1 is a ratio of 1/1.10=0.91.
Another report is Gallagher, K. and Nelson P. Manufacturing Costs of Batteries for Electric Vehicles. In Lithium-Ion Batteries: Advances and Applications, Pistoia, G. Ed.; Elsevier Science & Technology Book, 2014; p 103, which teaches the negative electrode thickness is determined by its specific reversible capacity and the designed excess capacity to prevent lithium plating during charging. The report teaches a ratio of 1.25 negative to positive reversible capacity ratio (N/P ratio) for cells with graphite negative electrodes. Lithium titanium oxide (LTO) negative electrode-based cells are designed at a 1.1 N/P ratio because of the minimal possibility of lithium metal deposition.
There is a need for improved secondary electrochemical storage devices and maximizing the volumetric or gravimetric energy density of said devices remains a clear design goal.